Arc welding wire of high feeding performance and wire drawing method

ABSTRACT

Disclosed is a wire for arc welding of high feedability having a hardness deviation of less than 18 between a central portion and a surface of a cross section of the wire, and a hardness deviation of less than 15 between each interval of 200 mm in a longitudinal direction when measured by an Hv1 hardness tester. The hardness deviation of the wire is adjustable through control of the area, in which the wire is in contact with dies. The present invention is characterized by adjusting the hardness deviation of the wire by adjusting the contact area ratio defined by the following formula. 
     
       
         Contact area ratio=Reduction contact ratio (Reduction contact area/Cross section area of an incoming wire)+Correction contact ratio (Correction contact area/Cross section area of an outgoing wire)

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wire for arc welding, and inparticular, to a wire for arc welding, which has enhanced feedability byuniformly distributing residual stress of the final wire product.

Wire acts as a filler metal for the mechanism of arc welding. To bespecific, wire is wound around a spool or a pail pack for welding, andpasses through a feeding roller and a welding torch cable in welding.The wire is then melted by an electric arc heating so as to be weldedwith a base metal. Therefore, it is important to provide a highfeedability for stable welding to the wire. Further, in the light of therecent welding work seeking automation and high efficiency, it ismandatory to provide a stable feeding of wire in a rapid feedingvelocity. Thus, the demand for enhancing feedability of wire isincreasing.

2. Description of the Related Art

In a variety of wires including the one for arc welding, an initial rodpasses through dies of diverse sizes, and is drawn to be a final wireproduct after undergoing steps of reducing diameter thereof to bethinner.

In the wire drawing process, the factors related to wire feedability maybe a wire drawing schedule in accordance with the diameter reducing ratefor drawing the wire having a desired diameter, distribution of internalstress through adjustment of deviation of the tensile strength and theelongation of wire, straightness of wire, etc. Of those, an uniformdistribution of internal stress of the wire is a critical factor to beconsidered in enhancing the wire feedability.

The conventional method of controlling a wire drawing process to enhancefeedability of the wire was limited to considering the diameter reducingrate only to reduce a diameter of the wire or an uniform distribution ofthe internal stress by adjusting the tensile strength or the elongationof the wire.

As the drawing of wire is repeated in the wire drawing process, however,the external portion of the wire, i.e., the outer surface of the wire,with which the dies are in contact, becomes denser than central portionof the wire and is hardened. As the surface of the wire is hardened, itis impossible to draw a wire, and the distribution of residual stressbetween the outer surface and the central portion of the wire becomesirregular. Therefore, the conventional control focused on a mereadjustment of the wire drawing schedule in accordance with the diameterreducing rate or an adjustment of the tensile strength has a limit inachieving a uniform distribution of residual stress between the outersurface and central portion of the final wire product.

Further, the hardness of the outer surface of the wire resulting fromthe repeated drawing thereof causes an abrasion of the dies, which arein contact with the wire, and causes irregular and damaged surface ofthe drawn wire, thereby lowering the quality of a final-wire product andinterrupting a smooth feeding of the wire in the course of welding.

The abrasion of dies caused by contacting with the wire having ahardened outer surface results in an irregular contact area, which is incontact with the wire, and further results in an irregular distributionof the residual stress in the longitudinal direction of the final wireproduct. Accordingly, when the wire passes through a feeding roller anda welding torch cable in the course of welding, the load is partiallyconcentrated, thereby causing failure of wire feeding because ofentanglement and twist of the wire.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to enhance afeedability of a wire for arc welding by uniformly distributing aninternal stress of the wire by adjusting the hardness difference in aradial and a longitudinal direction of the wire in the wire drawingprocess.

Another object of the present invention is to provide a wire for arcwelding having a uniform distribution of residual stress of the wire bycontrolling an area of the wire contacting with dies, and by reducingthe hardness difference of the wire.

Still another object of the invention is to provide a method of drawinga wire for arc welding that divides the final wire drawing step in anordinary wire drawing process into two steps, whereby a hardnessdifference between a central portion and an outer surface of the wire isreduced through adjustment of the contact angle between the wire and thedies in the first step, and the hardness deviation in the longitudinaldirection of the wire is reduced through adjustment of the length of abearing part in which the wire is corrected.

To achieve the above objects, there is provided a wire for arc weldingmanufactured by firstly drawing a wire material, heat treating the drawnwire material for work hardening of it, secondary drawing the heattreated wire material, heat treating the secondary drawn wire materialfor removing internal residual stress of it, and finally drawing theheat treated wire material, wherein the final drawing step is carriedsuccessively out by a first die having a diameter reducing contact partof a smaller contact angle and a shorter bearing part for actuallyreducing a diameter of the wire to be worked, and a second die having alonger diameter correcting contact part than that of the first die foractually correcting the diameter of the wire, the second die beingdisposed in series with the first die, whereby a hardness differencebetween a central portion and an outer surface of the wire is less than18 and a hardness difference between portions at intervals of 200 mm inthe longitudinal direction is less than 15, when the hardness ofrespective wire portions is measured by Vickers Hardness tester.

The differences of hardness values are adjusted to the values when wirecontact area rate defined the following formula is limited within arange of 3 to 3.5:

wire contact area rate=a diameter reducing contact rate+a diametercorrecting contact rate

diameter reducing contact rate=area of part for reducing a diameter ofthe wire/cross section area of the wire incoming into the first die

diameter correcting contact rate=area of the part for correcting adiameter of the wire/cross section area of the wire drawn from thesecond die.

As a technical concept to achieve the above objects, there is alsoprovided a method of drawing a wire for arc welding to have a desireddiameter, method for manufacturing a wire for arc welding, whichcomprises steps of firstly drawing a wire material, heat treating thedrawn wire material for work hardening it, secondary drawing the heattreated wire material, heat treating the secondary drawn wire materialfor removing internal residual stress of it, and finally drawing theheat treated wire material, wherein the final drawing step includessteps of reducing a hardness difference in a radial direction of thewire using a first die having an area reduction contact part of asmaller contact angle and a shorter bearing part, and reducing ahardness difference in a longitudinal direction of the wire using asecond die having a longer diameter correcting contact part than that ofthe first die and being disposed in series with the first die.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1A is a transversal section view of a wire passing through afeeding roller;

FIG. 1B is a longitudinal section view of a wire passing through afeeding roller;

FIG. 2 is a diagram illustrating a reduction contact area and acorrection contact area when a wire passes through dies;

FIG. 3 is a diagram illustrating two divided steps of a final wiredrawing process according to the present invention; and

FIG. 4 is a diagram illustrating a method of testing a feedability of awire (2-turn feedability test) according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

FIG. 2 is a diagram illustrating a diameter reducing contact part 20 forreducing a diameter of a wire W and a diameter correcting contact part200 (a bearing part) for correcting the diameter of the wire W when thewire W passes through die D.

The contact area between the wire W and the die D is mainly determinedby the following factors: i) a contact area of the die D with the wire Win which actual diameter reduction of the wire W is performed; and ii)straightness of the wire and a contact area of the wire W with thebearing part 200 in accordance with the straightness. The diameter ofthe W wire is corrected by the bearing part 200 so as to have anenhanced straightness.

In case of the factor i), when there is excessively small the diameterreducing contact part 20 for actually reducing the diameter of the wireW, the residual stress deviation in a radial direction of the wire Wbecomes greater. This results in a greater difference of hardnessbetween one outside and the other outside of the wire W. As aconsequence, the wire W is twisted if it fails to resist continuouspartial load (refer to FIGS. 1A and 1B) laid thereon when the wire Wpasses through a feeding roller in the course of welding, therebyresulting in vibration of the tip of the wire W that might cause an arcinstability. Further, when there is excessively large the diameterreducing contact part 20 for actually reducing the diameter of the wireW, a partial work hardening occurs, thereby lowering the quality ofsurface of the wire W. In the worst case, the partial stress deviationbetween the inside (central portion) and outside of the wire W becomesgreater, thereby disabling drawing of the wire W.

In case of the factor ii), when the contact area of the wire W with thebearing part 200 is excessively small, the deviation of the internalstress in the longitudinal direction of the wire W becomes greater, andthe feeding of the wire W is not smoothly performed. As a consequence,the wire W fails to bear continuous partial load acting thereon when thewire W passes through a feeding rollers 1, and is entangled or twisted,thereby causing a departure of the wire W from the feeding rollers 1 ora bending of the wire W. Thus, the wire W is likely to be deformed afterpassing through the feeding rollers 1 or a cable in the welding process.The deformed wire W has no straightness after passing through a contacttip, thereby causing a defect in welding (i.e., a bead meandering).

The conventional method of controlling such a deviation of internalstress employed a manner of controlling the tensile strength and theelongation of a wire product by a stable diameter reducing rate.However, this manner has a limit to controlling a stress of the outersurface of the wire W receiving a load in the feeding as well as of thecentral portion of the wire W receiving the load from the outer surface.

Under these circumstances, the inventors of the present invention havediscovered and conceived the fact that the internal stress of the wirecan be uniformly distributed by controlling the total contact area. Thetotal contact area can be obtained by summing an area of the diameterreducing contact part 20 for actually reducing the diameter of the wireW when the wire W passes through two dies D1 and D2, and an area of thediameter correcting contact part 200 for actually correcting thediameter of the wire W.

The inventors of the present invention have discovered another fact thatdistribution of the residual stress of such a final wire product isclosely related to the hardness deviation in the radial and thelongitudinal directions of the wire. To be specific, the inventors havediscovered that the physical property of the wire itself relating toenhancement of the feedability of the wire is affected by an uniformdistribution of the internal stress in accordance with the reduction ofthe hardness deviation in the radial and the longitudinal directions ofthe wire, and that the reduction of the hardness deviation can beachieved by controlling a wire contact area to dies D1 and D2 within apreferable range. With respect to control of the contact area, it isimportant to control the wire contact area in the final drawing steps ofthe wire drawing process.

The wire drawing process is usually performed in multiple drawing stepsto produce a wire having a smaller diameter. However, all the internalstress residing in the wire in the multiple drawing steps is reflectedin the wire immediately before taking the final drawing step.Accordingly, it is critical to control the residual internal stress ofthe wire in the final drawing step.

To be specific, the final drawing step is divided into two steps asshown in FIG. 3. In the first step, the contact angle of the wire W witha first die D1 is lessened to reduce the hardness deviation in theradial and the longitudinal directions of the wire W and to prevent thevibration of tip of the wire W by distortion of the wire in welding. Inthe second step, the length of the bearing part of a second die D2,i.e., the length of the diameter correcting contact part 200 is longerto reduce the hardness deviation in the longitudinal direction of thewire W and subsequently to prevent defect of welding (bead meandering)by bending or twisting to the wire when passing through a cable.

The present invention is characterized in that the residual stress ofthe wire is drastically decreased by controlling the wire contact arearate to be within the range of 3-3.5, whereby the hardness deviation inthe radial and the longitudinal directions of the wire are reduced. Thewire contact area rate is defined by summing a value of the diameterreducing contact rate and a value of the diameter correcting contactrate in two dies D1 and D2.

The following is a detailed description of a preferred embodiment of thepresent invention.

Embodiment

To study a relation between the hardness difference in the radial andthe longitudinal directions of the wire and a weldability, weldabilitywas evaluated based on a wire for stainless, which is relatively morestressful to work hardening in the drawing process.

TABLE 1 Hardness deviation (Hvl) Radial Longitudinal Direction DirectionFeeding Classifi- Contact of the of the Load cation Area Ratio Wire Wire(A) Remarks Present Invention 1 3.4 10.5 10.4 1.8 2 3.3 11.0 5.0 1.5 33.1 9.5 16.1 2.2 4 3.5 12.8 7.3 1.7 5 3.0 18.5 10.5 2.1 ComparativeExample 6 2.1 20.0 16.4 2.6 7 2.5 18.5 15.5 2.4 8 2.3 19.5 16.0 2.5 92.4 18.4 15.8 2.5 10  14.1 3.6 Heat Treated Wire

The diameter of the wire is reduced from 5.5 mm to 1.2 mm, and the wireis applied to steel such as AWS ER 309, JIS Y309. The feedability wastested in 2-turn form as shown in FIG. 4, and the welding condition was190A-220V. The wire for arc welding manufactured by firstly drawing awire material, heat treating the drawn wire material for work hardeningit, secondary drawing the heat treated wire material, heat treating thesecondary drawn wire material for removing internal residual stress ofit, and finally drawing the heat treated wire material. The final stepof wire drawing step was divided into two steps, and the hardness wasmeasured by means of a Vickers hardness tester (hereinafter, referred toas an “Hv1”) with respect to each wire after changing the wire contactarea rate in each of the wire drawing steps (of the final wire drawingprocess).

The heat treatments are performed after the first drawing and before thefinal drawing. The heat treatment performed after the first drawing isto release the work hardening of the drawn wire for the next drawingbecause stainless steel is stressful to work hardening. The heattreatment performed before the final drawing is to minimize anduniformly to distribute the internal residual stress of the final wireproduct because a distribution of the residual stress of the incomingwire is as much significant as releasing the stress of the wire whenpassing through two dies D1 and D2. The heat treatment performed beforethe final wire drawing is also important because even if the stress isreleased more or less after the first drawing, the residual stressdistribution in the wire can scarcely be achieved to a desirable extentdue to the continued second drawing that causes irregular distributionof the internal residual stress.

The hardness deviation in the radial direction of the wire was obtainedby measuring the hardness of the central portion and the outer surfaceof the wire, while the hardness deviation in the longitudinal directionof the wire was obtained by consecutively measuring the hardness fivetimes at intervals of 200 mm and by arithmetically averaging themeasured values (arithmetical average value of three test samples).

As described above, the final drawing (i.e., the third drawing) step wasdivided into two steps. In the first drawing step, the diameter reducingcontact area was controlled through adjustment of the contact angle ofthe wire with the first die D1. In the second drawing step, the diametercorrecting rate, i.e., the correction contact area in the step ofcorrecting the diameter of the drawn wire, is controlled throughadjustment of the length of the diameter of the diameter correctingcontact part of the second die D2, and the hardness deviation in theradial and the longitudinal directions of the wire are reduced touniformly distribute the residual stress of the wire. In other words,the hardness deviation in the radial direction of the wire is reduced bylessening the contact angle of the wire with the dies to preventvibration of the tip of the wire caused by twisting of the wire in thefirst step of the welding process. In the second step, the hardnessdeviation in the longitudinal direction of the wire is reduced byincreasing the length of the bearing part 200 of the second die D2, inwhich the diameter of wire is corrected, to prevent defect of welding(bead meandering) caused by bending or twisting of the wire when passingthrough a cable.

As shown in the Table 1 above, the feeding load is most stable in case ahardness difference between the central portion and the outer surface ofthe wire is less than 18 and a hardness difference between portions atintervals of 200 mm in the longitudinal direction is less than 15, whenthe hardness of respective wire portions is measured by Vickers Hardnesstester. In the case of the Examples 1, 2 and 4 showing the hardnessdeviation in the radial and the longitudinal directions of the wire tobe within the preferable range, the feedability becomes higher and thearc becomes stable as the feeding load becomes lower. In case of theExamples 3 and 5, however, anyone of the hardness deviation values inthe radial direction and the longitudinal directions is out of thepreferable range, so that the feeding load tends to be higher. Thisphenomenon is because the wire contact area rate is of summation of thediameter reducing contact rate and the diameter correcting contact rate.This means that a stable feedability can be obtained not only when thetotal wire contact area rate is controlled within a preferable range butalso when the diameter reducing contact rate and the diameter correctingcontact rate as well are controlled within a preferable range.

In a 2-turn welding test of a wire, arc becomes unstable when thefeeding load is about 2.1. When the feeding load is higher than 2.1,however, welding can be performed but welding cannot be consecutivelyperformed due to instability of arc.

As described above, the feedability of the wire can be enhanced bycontrolling the hardness difference between the central portion and theouter surface of the wire to be less than 18 and the hardness differencebetween portions at intervals of 200 mm in the longitudinal direction tobe less than 15, when the hardness of respective wire portions ismeasured by Vickers Hardness tester.

The present invention provides a wire for arc welding with uniformdistribution of residual stress of the wire by controlling the contactarea of the wire with dies to be within a preferable range so as toreduce hardness deviation of the wire. As a specific method, the presentinvention provides a wire drawing method by dividing the final drawingstep into two steps.

While the invention has been shown and described with reference to acertain preferred embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A solid wire for arc welding manufactured byfirstly drawing a solid wire material, heat treating the drawn wirematerial for work hardening it, secondary drawing the heat treated wirematerial, heat treating the secondary drawn wire material for removinginternal residual stress of it, and finally drawing the heat treatedwire material, wherein the final drawing step is carried successivelyout by a first die having a diameter reducing contact part of a smallercontact angle and a shorter bearing part for actually reducing adiameter of the wire to be worked, and a second die having a longerdiameter correcting contact part than that of the first die for actuallycorrecting the diameter of the wire, the second die being disposed inseries with the first die, whereby a hardness difference between acentral portion and an outer surface of the wire is less than 18 and ahardness difference between portions at intervals of 200 mm in thelongitudinal direction is less than 15, when the hardness of respectivewire portions is measured by Vickers Hardness tester.
 2. The wire forarc welding of the claim 1, wherein the differences of hardness valuesare adjusted to the values when wire contact area rate defined thefollowing formula is limited within a range of 3 to 3.5: wire contactarea rate=a diameter reducing contact rate+a diameter correcting contactrate diameter reducing contact rate=area of part for reducing a diameterof the wire/cross section area of the wire incoming into the first diediameter correcting contact rate=area of the part for correcting adiameter of the wire/cross section area of the wire drawing from thesecond die.
 3. A method for manufacturing a solid wire for arc welding,which comprises steps of firstly drawing a solid wire material, heattreating the drawn wire material for work hardening it, secondarydrawing the heat treated wire material, heat treating the secondarydrawn wire material for removing internal residual stress of it, andfinally drawing the heat treated wire material, wherein the finaldrawing step includes steps of reducing a hardness difference in aradial direction of the wire using a first die having an area reductioncontact part of a smaller contact angle and a shorter bearing part, andreducing a hardness difference in a longitudinal direction of the wireusing a second die having a longer diameter correcting contact part thanthat of the first die and being disposed in series with the first die.4. A solid wire for arc welding manufactured by firstly drawing a solidwire material, heat treating the drawn wire material for work hardeningit, secondary drawing the heat treated wire material, heat treating thesecondary drawn wire material for removing internal residual stress ofit, and finally drawing the heat treated wire material, wherein thefinal drawing step is carried successively out by a first die having adiameter reducing contact part of a smaller contact angle and a shorterbearing part for actually reducing a diameter of the wire to be worked,and a second die having a longer diameter correcting contact part thanthat of the first die for actually correcting the diameter of the wire,the second die being disposed in series with the first die, whereby ahardness difference between a central portion and an outer surface ofthe wire is less than 18 and a hardness difference between portions atintervals of 200 mm in the longitudinal direction is less than 15, whenthe hardness of respective wire portions is measured by Vickers Hardnesstester, in addition the two differences of hardness values are adjustedto the values when wire contact area rate defined the following formulais limited within a range of 3 to 3.5, wire contact area rate=a diameterreducing contact rate+a diameter correcting contact rate.